The present invention relates to an objective lens which is installed in an optical information recording/reproducing device for recording information to and/or reproducing information from multiple types of optical discs differing in recording density.
There exist various standards of optical discs (CD, DVD, etc.) differing in recording density, protective layer thickness, etc. Meanwhile, new-standard optical discs (HD DVD (High-Definition DVD), BD (Blu-ray Disc), etc.), having still higher recording density than DVD, are being brought into practical use in recent years to realize still higher information storage capacity. The protective layer thickness of such a new-standard optical disc is substantially equal to or less than that of DVD. In consideration of user convenience with such optical discs according to multiple standards, the optical information recording/reproducing devices (more specifically, objective optical systems installed in the devices) of recent years are required to have compatibility with the above three types of optical discs. Incidentally, in this specification, the “optical information recording/reproducing devices” include devices for both information reproducing and information recording, devices exclusively for information reproducing, and devices exclusively for information recording. The above “compatibility” means that the optical information recording/reproducing device ensures the information reproducing and/or information recording with no need of component replacement even when the optical disc being used is switched.
In order to provide an optical information recording/reproducing device having the compatibility with optical discs of multiple standards, the device has to be configured to be capable of forming a beam spot suitable for a particular recording density of an optical disc being used, by changing a NA (Numerical Aperture) of an objective optical system used for information reproducing/registering, while also correcting spherical aberration which varies depending on the protective layer thickness changed by switching between optical discs of different standards. Since the diameter of the beam spot can generally be made smaller as the wavelength of the beam gets shorter, multiple laser beams having different wavelengths are selectively used by the optical information recording/reproducing device depending on the recording density of the optical disc being used. For example, for DVDs, a laser beam with a wavelength of approximately 660 nm shorter than approximately 790 nm for CDs is used. For the aforementioned new-standard optical discs, a laser beam with a wavelength still (e.g. so-called “blue laser” around 408 nm) shorter than that for DVDs is used in order to deal with the extra-high recording density.
In recent years, a technology for suitably converging a light beam onto a record surface of an optical disc being used by providing an annular zone structure for an at least one optical element (e.g., an objective lens) of an objective optical system has been brought into practical use. More specifically, the annular zone structure formed on a surface of the optical element is configured to have a plurality of annular zones divided by minute steps. Through the function of the annular zone structure, each of the multiple types of light beams having different wavelengths is suitably converged onto the record surface of each of the optical discs of different standards.
It is preferable that, the above described optical element has a function of correcting the spherical aberration caused when the wavelength of the laser beam being used shifts from a design wavelength due to individual differences between light sources or environmental variations, such as temperature variations. The term “design wavelength” means an optimum wavelength for recording and/or reproducing for each of the optical discs of different standards.
An example of the objective optical system having the compatibility with the three types of optical discs (e.g., CD, DVD and HD-DVD) is disclosed in Japanese Patent Provisional Publication No. 2007-4962A (hereafter, referred to as JP2007-4962A). The objective optical system disclosed in JP2007-4962A has a phase shift structure designed to achieve a high use efficiency of light when an optical disc of the new-standard, such as HD-DVD, is used. In addition, the objective optical system is configured to prevent decrease of the S/N ratio of a focus error signal caused by undesired diffraction order light (i.e., flare light) during use of CD by controlling the longitudinal chromatic aberration during use of CD. The term “use efficiency of light” in this specification means a ratio of the light amount defined on a record surface of an optical disc being used with respect to the amount of light emitted from a light source.
The term “undesired diffraction order light” means light of a diffraction order not used for the information recording and the information reproducing. On the other hand, “normal diffraction order light” means light of a diffraction order used for the information recording and the information reproducing.
It is not necessarily required to increase the use efficiency of light for the new-standard optical disc to a considerably high level at the expense of the use efficiency of light for the other types of optical discs. In other words, it is possible to design an objective optical system having the compatibility with the three types of optical discs (e.g., CD, DVD and the new-standard optical disc) based on a design concept different from that of JP2007-4962A. For example, in order to ensure the stability of the recording function or the reproducing function for an optical disc having a relatively low recording density, such as CD, a designer may increase the use efficiency of light for CD to a certain extent.
However, increasing the user efficiency of light for CD leads to occurrence of the undesired diffraction order light during use of the new-standard disc and decrease of the S/N ratio of the focus error signal. As in the case of the objective optical system disclosed in JP2007-4962A, it is possible to prevent decreasing of the S/N ratio of the focus error signal by finely adjusting a convergence point of the undesired order diffraction light. However, it should be noted that the new-standard optical disc may have a multilayer structure formed of a multiple record layers. Therefore, if an objective optical system is designed simply based on the design concept of JP2007-4962A, the undesired diffraction order light may converge onto a record layer positioned near a record layer onto which the normal diffraction order light converges. In this case, the focus error signal may be badly affected.